In the related art, an electronic device with an electronic element mounted on an electronic element mounting substrate is known. As an electronic element mounting substrate used in such an electronic device, a substrate including a first wiring substrate, a metal plate bonded to a lower surface of the first wiring substrate, and an electronic element connection pad disposed on an upper surface of the first wiring substrate is exemplified. The electronic device is configured such that the electronic element is mounted on the electronic element mounting substrate and a cover or the like is disposed on the upper surface of the electronic element mounting substrate. In such an electronic device, the electronic element is mounted on a recessed portion formed by an upper surface of the metal plate and an inner surface of the first wiring substrate, and an external circuit or the like is electrically connected to an external circuit connection electrode which is disposed on an upper surface of the first wiring substrate, for example (see Japanese Unexamined Patent Publication JP-A 2006-303400).
In general, there is a difference in thermal expansion rates between the first wiring substrate formed of an insulating layer and the metal plate, and the thermal expansion rate of the metal plate is greater than that of the first wiring substrate. Therefore, if the metal plate is disposed on the lower surface of the first wiring substrate forming the electronic element mounting substrate and the electronic element is mounted on the upper surface of the metal plate, heat generated when the electronic element operates causes thermal stress between the first wiring substrate and the metal plate. There is a concern that the thermal stress concentrates on a bonding member which bonds the first wiring substrate to the metal plate and cracking or peeling-off occurs. Therefore, it has been considered to reduce the bonding area between the metal plate and the first wiring substrate. Thus, an electronic element mounting substrate in which a peripheral portion of the metal plate is located inside a peripheral portion of the first wiring substrate in a top transparent view is known.
In general, force is applied in a direction from the upper surface toward the lower surface of the first wiring substrate in a process of mounting a component such as a lens case body, the cover, or an electronic component on the upper surface of the first wiring substrate or in a process of performing wire bonding on the upper surface of the first wiring substrate. However, in the case of the electronic element mounting substrate with the aforementioned configuration, there is no object to be supported but only a space corresponding to the thickness of the metal plate in a region where the first wiring substrate and the metal plate are not overlaid. Therefore, there has been a concern that bending stress starting from the peripheral portion of the metal plate occurs in the first wiring substrate and cracking, breakage, or the like occurs in the first wiring substrate due to the application of the force in the direction from the upper surface to the lower surface of the first wiring substrate. Also, there is a requirement for reducing the thickness of the first wiring substrate. The concern of the occurrence of cracking, breakage, or the like in the first wiring substrate has become more serious due to the decrease in thickness.
An object of the invention is to provide an electronic element mounting substrate capable of suppressing occurrence of cracking and breakage in the first wiring substrate which is overlaid with the peripheral portion of the metal plate in a top transparent view and capable of realizing the decrease in the entire thickness even if a second wiring substrate is connected, and an electronic device using the electronic element mounting substrate.